Fasteners of multiple designs, including nuts, screws, clips, rivets and many other varieties, have been made available depending on the particularly encompassed application. A common feature thereof lies in the particularly stringent requirements associated herewith.
Indeed, during their life, any fasteners, included unthreaded and threaded fasteners, are notably submitted to harsh mechanical conditions: they are stretched, twisted and bent. Fasteners are further generally submitted to various aggressive environmental, such as vibrations, thermal cycles and/or chemical attacks, which can alter their mechanical performances over time, and, in the worst scenarios, cause fasteners to be literally “lost”.
A class of fasteners of particular importance is threaded fasteners that face with additional specific problems. When threaded fasteners are tightened, we pump energy into them, and, after we let go, this energy is held therein by friction constraints. Typically, these ones are concentrated to a large extent in the fastener threads, which yet often represent the most delicate portions of the fastener as the result of their fineness. Aggressive environmental factors as above recited can cause threaded fasteners to loose all their preload and literally be lost (“loosening” problem). Somewhat related to loosening is thread stripping, another failure which occurs when threaded fasteners are over-tightened; thread stripping is characterized by a deformation (alteration) of the fastener threads, typically resulting in a decrease of performance.
Certain fasteners, including certain unthreaded and threaded fasteners, can have complex designs, and shaping/machining them from an appropriate material can be a tough matter. In case of threaded fasteners, forming fine and regular threads has proved to be particularly uneasy.
Material selection is crucial for fasteners. Metal has remained for several tens of years the only suitable choice, because no plastic material was able to provide the required level of mechanical properties, in particular a high elongational strength, a high flexural strength and, last but not least, a high torsional strength (or torque). Indeed, for many fasteners, including but not limited to threaded fasteners, load bearing ability is typically a function of the shear or torsional strength of the material of construction.
Metal fasteners present however a certain number of disadvantages. Metal fasteners are heavy; in contrast, certain applications, in particular self-propellant vehicles, and more particularly aircrafts, ask for light-weight materials. Metal fasteners are in general prone to corrosion; in addition, when dissimilar metal are joined together, galvanic corrosion can occur. Metal fasteners are electrically conductive. Machining complex shapes from a metal, as well as forming fine and regular threads, is a tough matter.
The development of engineering composites, such as carbon fiber-reinforced polyetheretherketone (PEEK) has well offered some viable alternatives to not very demanding fasteners applications. Adding fiber reinforcement increases well tensile and flexural properties, but has little effect on the shear properties. In addition, fiber reinforcement reduces tensile elongation, a measure of practical toughness. In addition, when fasteners are produced using the injection molding process with fiber reinforced materials, property variations due to flow direction occurs. In addition, when the fasteners are threaded, it is unlikely that the threads contain much fiber reinforcement, because they are relatively thin and perpendicular to the usual flow direction.
An important progress was achieved when the use of rigid rod polyphenylenes was proposed for the first time for the manufacture of unthreaded fasteners. Then, EP 2 014 251 describes a pin for securing the position of a part of a body (for example, a head) for medical procedures, which is made of an optionally substituted polyparaphenylene (unkinked rigid rod polyphenylene of the 1st generation), such as:
wherein R and R′ are substituents such as —C(═O)C6H5.
TECAMAX® SRP polyphenylene, commercially available from Ensinger, is alleged to be such a material. As the result of their rigid molecular structure, the polyparaphenylenes of EP 2 014 251 allow indeed for much higher torque than conventional composite polymer materials (e.g. carbon fiber-reinforced PEEK). No reinforcing fibers are needed; homogeneous materials have distinct advantages with regard to the uniformity of mechanical properties. However, neither the proposed rigid-rod polyphenylenes of the 1st generation nor, as a matter of fact, the slightly kinked rigid-rod polyphenylenes of next generation (as notably proposed by SOLVAY ADVANCED POLYMERS, L.L.C. as PrimoSpire® PR-120) are fully satisfactory for making the pins of concern. Such polyphenylenes can be qualified as “not very satisfactory” for very demanding applications, as certain unthreaded fasteners are notably as the result of their design (shape, thickness, etc.), and as most of threaded fasteners are as the result of the mandatory presence of threads. For said very demanding fastener applications, there is still a need for a polymer material that would provide a higher tensile elongation (a measure of practical toughness). Another problem, which can even be more acute for certain designs than the previous one, results from the intrinsic rigid nature of the so-proposed polyarylenes of the 1st two generations: shaping them into articles having complex shapes or with a very low thickness by melt processing techniques such as injection molding or extrusion, remains difficult, as it was originally the case for metal.
There is thus an important need for fasteners exhibiting a confluence of characteristics including high torque, high practical toughness (high tensile elongation), high elongational strength, high stiffness, high chemical resistance, light weight, and which can be easily formed by melt-processing techniques, such as extrusion or injection-molding, including when the parts of concern have a complex shape and/or very thin portions (e.g. threads, or when an extruder with small orifices must be used).